[Billy]

Definitely sounds like your FETs aren’t biased correctly

Try biasing the drains to 5v you won’t get 5v on them all there is a post somewhere if you search boogie 57 where I’ve noted my voltages

Here it is found it, from the post;

My Q3 is biased at 8.4v and works fine the source resistance change will affect it a little, 1, 2 and 4 biased to 5v and Q5 to around 8v

You’ll notice Q3 and 5 have no source bypass cap so gain is set by Q1, 2 and 4 making them important to bias at 5v ish

Boogie 57 – Gain issue & Biasing (solved)

The only thing I can see that’s incorrect in your build is R11 3K9 in the build doc, yours looks like 39K this would definitely affect gain especially if you’ve built it stock and used a 1M gain pot

What I’d do is make R11 100K use an A250K for the gain pot with a resistor to ground on lug 1 to stop it cutting out completely 8K2 worked for mine

If you look at the linked post youll see this makes the gain more gradual across the pot so you may as well kill two birds with one stone

[Billy]

You’re just measuring the resistance along that trace

So it’s all good as you can see on the schematic there’s only R3 1M between VB and IC1 pin 3

Post an image of the solder side of the pcb and visually check it for any solder bridges or joints that look ball like, grey or crumbly and reflow any dodgy looking joints

Everything between VB and pin 3 seems OK

[Billy]

Check resistance between VB the junction of R23 / 24 and IC1 pin 3 without powering the circuit

[Billy]

Yes to all your questions and I’d personally use 100n for the capacitors

[Billy]

Try swapping the op amps around to make sure IC1 isn’t faulty it doesn’t appear to be with your other voltage readings but worth a check

Reflow IC1 socket solder joints obviously remove the op amp before you do it

 

 

 

[Billy]

Yes late to the party again! Reflow the left side of R3

From your voltages the power supply side is doing what it should you’re just not getting your bias VB to the op amp input pin 3 through R3 obviously it could be something after R3 but hopefully  its just a poor solder joint on R3

[Billy]

Yeah apologies I meant the MKC IC1 pin 3, the buffer voltages look OK

I’ve been a wee bit busy so sorry for the delay

Pin 3 on the MKC is low but I notice you get a 1v increase with the IC removed  which is still a little low

What voltage do you get both sides of R3 you should have around 4v one side and 3v ish the other side

Check that and then try reflowing it’s solder joints

[Billy]

Check IC1 pin 3 voltage with the IC removed from the socket

[Billy]

You can’t really measure resistance accurately in circuit there may be series and parallel resistance connections

When measuring resistance in circuit it should not be powered and you should try to measure resistance from reference points in the circuit for example

To measure R22 value place your meter probes between IC2A pins 1 and 2 you can see from the schematic that between those 2 points there should only be around 100K resistance (we can actually see it is 100K from the colour bands on it)

From the colour bands I can see your values on both pcbs look correct although quite a few of the multiplier bands are hard to see whether they’re brown or red

Use the build doc populated board image to check all values are correct

Time for a quick review I think, your voltages are what’s thrown me I initially thought you had negative voltages with the dash –

What I think you’ve done is simply measured your ICs upside down and pin 4 is actually pin 8

Pin 1 of your ICs is bottom left so your voltages are fine if that’s the case

What are the voltages on the buffer pcb IC

Post an image looking down on both pcbs so that we can clearly see all components and wiring connections it’s difficult to check values and trace your connections with the images you’ve posted

We can’t quite see the connections to your stereo in jack make sure that’s correct and you haven’t accidentally wired your in wire to the ring instead of the tip connection

You can insert a jack and follow the connections around to their solder lugs like this

 

 

 

 

 

 

 

 

 

[Billy]

From your voltages there must be a power issue pin 4 should be 0v and pin 8 power

Are you using a centre negative power supply

[Billy]

Yes that’s correct is the ground lug bridging the middle battery lug

Are you numbering your IC pins like this

 

[Billy]

Can’t quite see how you’ve got the DC jack wired

Post an image so we can see the face of it and which lugs your 9v and ground wires go to

Because it’s buffered bypass you won’t get anything if its incorrectly wired from your voltages it looks like it may be inverted unless your pin numbers are incorrect this is how it should be connected

 

[Billy]

What transistor type do you have in Q3 it’s not an MP42B

What did you use to determine the transistor pinouts

All your component values and orientation that I can see look correct

I remember this from Reddit a while back I’m mcknib on there

 

[Billy]

I think Barry’s suggestion of increasing R2 would definitely reduce the signal level driving the 1st stage I’d try a 100K pot or trimmer wired as a variable resistor there or simply socket it and try different value resistors to see which value works best with your pickups

[Billy]

No it doesn’t mater you’re simply setting the bias voltage on the drain pin, sorry I thought pots meant the trimpots

[Billy]

Some good info here you want to aim for around 5v on the FET drain pins although Q3 etc as you can see here are higher

Boogie 57 – Gain issue & Biasing (solved)

 

[Billy]

It does sound like your pickups are too hot

If you know which circuit / part of the circuit the clipping starts at try lowering op amp feedback resistors like the 200K R3 in the add verb I’d try 100K

[Author]

Posts